Conventionally, as a focus mechanism changing the focal length or focus position of an optical system, the system of adjusting the focus by moving a lens has been widely used. However, this system needs a lens drive mechanism and thus has defects that the mechanism becomes complicated and needs comparatively high electric power for the lens-drive motor. Moreover, shock resistance is generally low. Therefore, as a focus mechanism that does not require the lens drive mechanism, such a system that adjusts the focus by changing the refractive index of a liquid crystal lens has been proposed (for example, see Patent document 1).
The liquid crystal lens for attaining this conventional system has a configuration of holding a liquid crystal layer between two glass substrates each of which is equipped with a pattern electrode and a common electrode. This pattern electrode has a core electrode and plural ring electrodes, and has a configuration that the core electrode and each of the ring electrodes are connected by a voltage drop resistor. To an extraction electrode that is connected to the core electrode but is insulated from each ring electrode, a variable resistor is connected via a power amplifier, and to an extraction electrode that is connected to a ring electrode (a peripheral electrode), variable resistor is connected via an amplifier. Furthermore, the alternating voltage supplied from the alternating power source connected in parallel with these variable resistors is lowered by the variable resistors.
Thus, voltage distribution is formed by the voltage signal applied to the extraction electrodes and the voltage drop resistors, and voltage distribution is formed on the liquid crystal layer. By adjusting each variable resistor, it becomes possible to generate various forms of voltage distribution on the liquid crystal layer.
As an autofocus (automatic focus) system for a video camera, a contour detection system is publicly known that extracts information corresponding to blurriness of an image directly from a photograph image signal and carries out hill-climbing control of the lens to minimize the blurriness (for example, see Non-patent literature 1). Various autofocus devices and the like using this hill-climbing control system have been proposed (for example, see Patent document 2, Patent document 3, Patent document 4, and Patent document 5).    Patent Document 1: Japanese Patent Publication No. 3047082    Patent Document 2: Japanese Utility Model Application Publication No. H2-44,248    Patent Document 3: Japanese Patent Publication No. 2742741    Patent Document 4: Japanese Patent Application Publication No. H1-15188    Patent Document 5: Japanese Patent Application Publication No. H2-11068    Non-patent Literature 1: Technical Report of Institute of Television Engineers of Japan, Nov. 29, 1982, pp. 7-12, titled “Contour Detection Autofocus System” by Kentaro Hanma and four others